WDPN31 PGTW 110300
MSGID/GENADMIN/NAVPACMETOCCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W WARNING NR 09//
RMKS/
1. FOR METEOROLOGISTS.
2. PROGNOSTIC REASONING FOR 110000Z TO 160000Z OCT 2006.
   A. TROPICAL STORM (TS) 21W (SOULIK), LOCATED APPROXIMATELY 400 NM
SOUTHEAST OF IWO JIMA HAS TRACKED WEST-NORTHWESTWARD AT 16 KNOTS
OVER THE PAST SIX HOURS. RECENT MULTI-SPECTRAL IMAGERY INDICATES
THAT THE LOW LEVEL CIRCULATION IS TO THE WEST OF THE CONVECTION,
SUPPORTING A RELOCATION 55 NM (APPROX) TO THE WEST.
   B. TS 21W IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF THE
SUBTROPICAL RIDGE (STR) CENTERED NORTH OF WAKE ISLAND. THE STR WILL
REMAIN TO THE NORTHEAST OF THE SYSTEM THROUGH TAU 36, CAUSING TS 21W
TO TRACK WEST-NORTHWESTWARD. AROUND TAU 48, THE SYSTEM WILL SLOW DUE
TO A WEAK STEERING ENVIRONMENT WITH COMPETING INFLUENCES OF THE STR
TO THE EAST-NORTHEAST, A MIDLATITUDE TROUGH TO THE NORTH, AND
ANOTHER STR TO THE WEST. THIS IS SUPPORTED BY THE AVAILABLE DYNAMIC
AIDS, WHICH ARE IN FAIR AGREEMENT THROUGH TAU 48. THE WEAK STEERING
ENVIRONMENT WILL CONTINUE THROUGH THE EXTENDED TAUS.
   C. TS 21W WILL INTENSIFY AT A LESS THAN CLIMATOLOGICAL RATE
THROUGH TAU 48 DUE TO RESTRICTED POLEWARD OUTFLOW BUT SUFFICIENT
EQUATORWARD OUTFLOW. THIS IS SUPPORTED BY THE AVAILABLE NUMERICAL
GUIDANCE. BEYOND TAU 48, AS THE SYSTEM SLOWS, CONSOLIDATION AND
IMPROVED OUTFLOW ALOFT WILL ALLOW TS 21W TO INTENSIFY AT A
CLIMATOLOGICAL RATE THROUGH THE EXTENDED TAUS. 
   D. CURRENT WIND RADII ARE BASED ON A 102018Z QUIKSCAT IMAGE AND
CLIMATOLOGY FOR AN AVERAGE TO LARGE-SIZED SYSTEM. FORECAST WIND
RADII ARE BASED ON CLIMATOLOGY FOR AN AVERAGE TO LARGE-SIZED
SYSTEM.
   E. CONSIDERABLE UNCERTAINTY EXISTS FOR THE TRACK OF THE SYSTEM
IN THE EXTENDED TAUS. THE MOST LIKELY SCENARIO IS RECURVATURE
OF THE SYSTEM DUE TO INTERACTION WITH A MIDLATITUDE TROUGH BETWEEN
TAU 96 AND TAU 120. THIS IS REFLECTED IN THE FORECAST TRACK AND
SUPPORTED BY THE MAJORITY OF THE OBJECTIVE AIDS. AS AN ALTERNATE
SCENARIO, A WEAKER OR SLOWER TROUGH MAY ONLY TEMPORARILY STAIR-
STEP THE TRACK POLEWARD BEFORE RETURNING TO A WESTERLY TRACK.
THIS ALTERNATE SCENARIO IS SUPPORTED BY A MINORITY OF THE AIDS,
CONSISTING OF AFWA MM5 AND GFDN. DURING THE EXTENDED TAUS,
INCREASED VERTICAL WIND SHEAR WILL INHIBIT FURTHER DEVELOPMENT AND
MAINTAIN THE INTENSITY.
FORECAST TEAM: CHARLIE//
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